Optical transceivers are key components in optical fiber-based telecommunications and data communication networks. An optical transceiver includes an opto-electronic light source, such as a laser, and an opto-electronic light receiver, such as a photodiode, and may also include various electronic circuitry associated with the laser and photodiode. For example, driver circuitry can be included for driving the laser in response to electronic signals received from the electronic system. Receiver circuitry can be included for processing the signals produced by the photodiode and providing output signals to the electronic system. Optical lenses are also commonly included.
Generally, there are two types of semiconductor laser devices: edge-emitting lasers and Vertical Cavity Surface Emitting Lasers (VCSELs). An advantage of VCSELs is that they can be tested economically at wafer level rather than chip level. Another advantage of VCSELs is their well-defined spot size, which promotes high coupling efficiency to optical fibers without the need to provide beam shape correction, thus facilitating economical packaging. Edge-emitting lasers also have advantages, such as robust reliability and high output optical power. Likely for these reasons, edge-emitting lasers remain the most commonly used semiconductor laser in high-speed optical transceivers. To test edge-emitting lasers, a wafer must be scribed and cleaved for single-chip testing. That is, wafers must be diced into bars, coated with highly-reflective (HR) or anti-reflective (AR) coatings, and then diced to single chips to be tested and selected. The process of testing edge-emitting lasers thus can be relatively uneconomical.
One way to reduce edge-emitting laser chip cost involves a process commonly referred to as etched facet. In an etched facet laser, the feedback mirrors are etched facets rather than cleaved facets. Etched facets facilitate coating the facet with highly reflective or, alternatively, anti-reflective layers at wafer-level, rather than at bar level. (See, e.g., Peter Vettiger, et al., IEEE Journal of Quantum Electronics, Vol. 27(6), June 1991, p. 1319.) A mirror-like etching profile is necessary to minimize reflection loss and provide a uniform coating thickness.
Edge-emitting lasers can have either a ridge waveguide structure or a buried waveguide structure. The process of fabricating a ridge waveguide structure is less complex than the process of fabricating a buried waveguide structure. For uncooled lasers, the active core layers are commonly made of aluminum-containing multiple quantum well (MQW) layers. In such a laser, a ridge waveguide structure is more advantageous than a buried waveguide structure because a ridge waveguide structure does not have an etched MQW region or suffer from aluminum oxidation.
The facets in an etched-facet laser are commonly etched by an inductively coupled plasma (ICP) process using a protection mask. This process is carried out after the formation of the waveguide. However, the presence of the ridge in such a ridge waveguide structure introduces two main difficulties for fabricating such a laser: facet quality and facet coating (particularly with regard to facet coating thickness).
With regard to the issue of facet quality in fabricating ridge waveguide lasers, it is almost impossible to have a smooth mirror-like etch profile on a ridge waveguide structure because the mask edge in the ICP process is disturbed by the non-planar ridge structure. The quality of facets produced in the ICP process depends strongly on mask profile. In S. C. Horst, et al., “High-reflectance dielectric mirrors deposited by Plasma-Enhanced Chemical Vapor Deposition on GaAs—AlGaAs Semiconductor Lasers with Inductively Coupled Plasma Etched Facets,” IEEE Photonics technology Letters, VOL. 12 (10), October 2000, p 1325-1327, a Benzocyclobuten (BCB) layer was used to first planarize the ridge, and then an SiO2 mask was deposited and defined on the BCB layer.
A key feature of the above-described process is a BCB etch-back process that transfers a non-planar ridge to a planar surface. However, the process is rather complicated and time-consuming, requiring a number of steps: BCB coating, thermal curing, and BCB etch-back, followed by SiO2 layer deposition, photolithography, SiO2 dry etch, ICP etch, and lastly removal of the BCB coating. It is difficult to provide a vertical mask profile such that the ICP etch process results in a vertical ridge profile. Furthermore, the removal of BCB residue can be problematic.
The other major issue in fabricating etched facet lasers with ridge waveguide relates to facet coating and, in particular, facet coating thickness control. For proper laser operation, either a highly reflective (HR) coating for Fabry-Perot (FP) lasers or an anti-reflective (AR) coating for Distributed Feedback (DFB) lasers must be applied. Such coatings are commonly applied by depositing SiO2/SiNx layer pairs on an entire wafer by Plasma-Enhanced Chemical Vapor Deposition (PECVD). However, the non-planar ridge structure causes a “shadowing effect,” which inhibits precise control of coating layer thickness near the ridge. For proper DFB laser operation, an AR coating layer having a ¼-wavelength thickness (or odd multiple thereof) is commonly applied.
As illustrated in FIGS. 1-3 (not to scale), a known ridge waveguide laser structure 10 includes etched windows 12 fabricated directly on the structure using a protection mask (not shown). The term “window” refers to a region etched down from the surface to the substrate. As described in further detail below, the facets defined by windows 12 are coated with either an HR or AR coating material, depending on whether the resulting structure is to be an FP laser or DFB laser. In this fabrication process, the “shadowing effect” of the ridge structure 14 commonly adversely impacts mask definition, ICP etch, and facet coating.
Multi-mask layers (not shown) are commonly used to fabricate a structure such as ridge structure 14 over an MQW layer 7. The first mask, which can be a BCB layer, is used with an etch-back process to planarize the ridge. Then, an SiO2 mask is deposited on the BCB surface. However, such a double mask hampers providing a vertical mask profile, which is fundamental to obtaining a vertical etch profile using the ICP process. Furthermore, such a multi-mask process is rather complicated and time-consuming, involving a number of steps: BCB coating, thermal curing, etch-back, SiO2 deposition on BCB, photolithography, reactive ion etching (RIE) SiO2 etch, BCB etch, ICP semiconductor etch, SiO2 removal, BCB removal, and facet coating. The last step before facet coating, i.e. removing the BCB layer, is often not completely effective, thus leaving BCB residues that hamper SiO2 or metal adhesion.
To provide good anti-reflective properties, the facets defined by windows 12 are commonly coated with one or more pairs of SiO2 and SiNx layers. Portions 9 and 19 of FIGS. 2A and 3A, respectively, are shown enlarged in FIGS. 2B and 3B. As illustrated in FIGS. 2B and 3B, the AR coating material can comprise, for example, two SiO2/SiNx coating layer pairs, i.e., four coating layers: a bottom or first coating layer 11 of SiNx; a second coating layer 13 of SiO2 over first coating layer 11; a third coating layer 15 of SiNx over second coating layer 13; and a top or fourth coating layer 17 of SiO2 over third coating layer 15. The total or combined thickness of the SiO2/SiNx coating layer pairs is commonly selected to be an odd multiple of ¼ wavelength and to be thick enough to help minimize parasitic capacitance. For these reasons, it is not uncommon to have two SiO2/SiNx coating layer pairs rather than only a single SiO2/SiNx coating layer pair.
Although such a 4-layer coating can advantageously provide good anti-reflective properties and low parasitic capacitance, such a 4-layer coating can pose problems for further device fabrication steps. In fabricating a DFB laser, further fabrication steps commonly include forming an electrical contact window on ridge structure 14 and applying a metal region to the electrical contact window. Reactive ion etching (RIE) is commonly used to form the electrical contact window. Because the RIE process etches SiO2 and SiNx coatings at different rates, the RIE process can leave the etched coating surface with a furrowed texture, as illustrated in FIG. 2C. This furrowed surface texture can cause layers to peel away from each other or otherwise hamper metal adhesion.
It would be desirable to provide a ridge semiconductor laser with high facet quality facet, stable facet coating yield, low parasitic capacitance, optimized coating design, and minimal fabrication challenges.